UPSC Civil Services Model Paper 4

When ethyne is burnt with air, it gives a yellow sooty flame which has much lower temperatures.
This is because ethyne is not burnt completely. So, statement 1 is correct.
When pure oxygen is used, the flame is cleaner, the combustion of ethyne is complete, and much higher temperatures can be used with lesser quantities of ethyne. So, statement 2 is correct.
Moreover, air contains many gases other than oxygen which may be undesirable in the process of welding.
This is why oxygen is used instead of air.